The present invention relates to the reproduction of capacitance disc records, and in particular to a circuit arrangement for applying high frequency energy to the electrode of a capacitance detection stylus.
It is known in the art that capacitance disc records comprise a resin formed of a copolymer of vinyl chloride and vinyl acetate by a compression molding to copy the microscopic pits formed on the surface of a stamper. The copied surface of the resin material is coated with a conductive layer which serves an electrode. On the conductive layer is an overlying layer of a dielectric material having a thickness of a few hundreds Angstrom units. A system of variable capacitance is formed by the conductive and dielectric layers and the electrode provided on a record stylus. The conductive layer may also be provided by mixing carbon particles with the copolymer prior to the molding process. The record stylus is typically constructed of a diamond on the rear face of which is provided the stylus electrode to which high frequency energy is applied for detecting the geometric variation as capacitance variation.
The dielectric layer is formed of a material both resistant to wear which might be caused by contact with the stylus and to a concentration of the high frequency energy. However, upon contact with the record surface, the stylus would exert a pressure of as high as several tens of ton per square centimeter if the tip of the stylus is inclined to the surface plane of the record with the attendant concentration of high frequency energy at the contact point of the stylus electrode. As a result, the surface of the dielectric layer may be severely damaged to such an extent that the underlyng conductive layer is revealed. In such instances, a short circuit path is established between the conductive layer of the disc and the stylus electrode producing a heavy current which results in impairment of the stylus tip and the disc surface which in turn results in the generation of errors in the output signal. Such impairments also occur at the instant the stylus is raised from the record surface. Since the impairment of the disc is likely to result in a dropout, the error of this type can be eliminated electronically by dropout compensation technique. However, the impairment of the stylus results in a poor carrier-to-noise ratio which cannot be compensated.